Click Solutions under the Develop section.

Click on the edit button on the required application.

Note: For Environment enabled application, please click Environment -> Edit.
Now the development mode will be displayed. Click on the Settings button on the top of the page.

In Settings page click on the Portal User Permissions option under Advanced settings section.

The Portal user permissions page will be displayed.

Hover the mouse over the permission set to be edited and click on the permission.

In the Edit permission tab that appears, make the required changes for the permission set. Then click Update.
